(a) Technical Field
The present invention relates to a photosensor and a display device including a photosensor.
(b) Disclosure of Related Art
Flat panel displays include a liquid crystal display (LCD), an organic light emitting display (OLED), and a plasma display panel (PDP).
The LCD is the most widely used flat panel display, which includes two panels and a liquid crystal layer having dielectric anisotropy and disposed between the two panels. The LCD applies electric field to the liquid crystal layer and controls the electric field to control transmittance of light passing through the liquid crystal layer, thereby displaying desired images.
Since the LCD is not a self-emissive display device, it includes a backlight unit for supplying light to the panels. However, the backlight unit gives a great contribution to the power consumption, and thus it is suggested that a photosensor be employed to control the backlight unit, particularly for portable devices such as mobile phones and notebook computers.
In the meantime, the LCD usually includes thin film transistors (TFTs) containing amorphous silicon and the amorphous silicon TFT generates photocurrent when exposed to light. Accordingly, the amorphous silicon TFT can be used for a photosensor since the photocurrent generated the amorphous silicon TFT depends on the amount of light, which can be recognized by human eyes.
FIG. 1 is a circuit diagram of a conventional photosensor used for a fingerprint identification system or a touch screen.
Referring to FIG. 1, a conventional photosensor includes two TFTs including a sensor TFT and a switch TFT and a storage capacitor. The sensor TFT is biased with a gate-off voltage Vgate_off for turning off the sensor TFT and generates a photocurrent when it is exposed to light and the magnitude of the photocurrent depends on the amount of light exposure. The storage capacitor C stores electric charges of the photocurrent to generate a photovoltage and the switch TFT outputs the photovoltage in response to a gate-on voltage Vgate_on.
However, the conventional photosensor shown in FIG. 1 may be easily interrupted by noise. For this reason, a device including the conventional photosensor may use the photosensor in an ON/OFF mode. Otherwise, the device further includes an expensive digital signal processing (DSP) chip for processing the output signal of the photosensor.